Multi-carrier high power amplifier using digital pre-distortion

ABSTRACT

A distortion correction technique for use with a high power amplifier (HPA) in a multi-carrier radio signaling system such as a cellular base station. Distortion correction is implemented by making use of a broadband digital composite signal input to the high power amplifier as a reference signal in a form of intermediate frequency (IF) distortion correction circuit. A multichannel synthesizer provides the broadband composite signal to a broadband digital radio which in turn provides an input to the (HPA). A portion of the output signal from the HPA is fed back through a radio frequency (RF) and intermediate frequency (IF) down-conversion stage that uses the same IF and RF local oscillators that were used to generate the input signal to the HPA. This feedback signal is fed to a predistortion processor together with a version of the composite digital signal. The predistortion processor may perform a first crude amplitude correction procedure by finding a difference between the HPA feedback signal and the composite signal, to provide an offset to be loaded into a look-up table which is disposed between the broadband digital synthesizer and the broadband digital radio,. Subsequent precise correction and distortion correction procedures are performed using the offset value.

FIELD OF THE INVENTION

This invention relates generally to power amplifiers and in particularto a distortion correction technique for a high power amplifier that isused to transmit a composite digital signal representing multiple radiochannels.

BACKGROUND OF THE INVENTION

The demand for cellular and other multichannel wireless communicationssystems has led to development of broadband transceiver systems that arecapable of processing many radio channels in parallel. While broadbandtransmission and reception of many radio channels in parallel throughthe use of sophisticated digital signal processing techniques providesan ultimate advantage of compact size and low price, this comes at thecost of more sophisticated, expensive, and sensitive radio equipment.

One such system component is the high power amplifier which must impartsufficient energy in the transmitted signal to close the communicationlink over distances of several miles to the mobile and portablesubscriber units. Since the HPA is transmitting many channels inparallel, it must operate as linearly as possible, and impart as littledistortion as possible in the resulting transmitted waveform.

For example, certain digital cellular protocols such as Global Systemfor Mobile Communications (GSM) specify that the intermodulationdistortion between adjacent channels must be at levels of -70 decibels(dB) or less.

DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION

It is an object of this invention to provide for distortion correctionin a high power amplifier which is intended for use in a multi-carrierradio signaling system such as a cellular base station that makes use ofbroadband processing techniques.

A further object is to permit the implementation of distortioncorrection in a multi-carrier power amplifier by making use of a digitalcomposite signal input to the high power amplifier as reference signalin a form of intermediate frequency (IF) distortion correction circuit.

SUMMARY OF THE INVENTION

Briefly, the invention is a wideband multi-carrier high power amplifier(HPA) that makes use of a broadband digital predistortion linearizationtechnique to provide high power amplification of complex multi-carrierwaveforms. In this arrangement, a multichannel synthesizer feeds abroadband digital radio, which in turn provides the input signal to theHPA. The predistortion technique feeds back a portion of the outputsignal from the HPA through a radio frequency (RF) and intermediatefrequency (IF) downconversion stage that uses the same IF and RF localoscillators that were used to generate the input signal to the HPA. Thisfeedback signal from the HPA is digitized and fed to a predistortionprocessor together with a version of the composite digital signalgenerated by the broadband digital synthesizer.

The predistortion processor may perform a first crude amplitudecorrection procedure by finding a difference between the HPA feedbacksignal and the composite signal, to provide an offset to be loaded intoa look-up table which is disposed between the broadband digitalsynthesizer and the broadband digital radio.

Subsequent precise correction procedure and distortion correctionprocedures are performed using the offset value.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention and its noveladvantages and features, reference should be made to the accompanyingdrawings in which:

FIG. 1 is a block diagram of a wideband power amplifier making use ofdigital pre-distortion intermediate frequency stage according to theinvention; and

FIGS. 2A through 2D are flow charts of the operations performed by apredistortion correction digital signal processor according to theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates the transmitter sub-assembly portion 10 of abroadband transceiver system (BTS) consisting of a baseband digitalsynthesizer 12, a digital predistorter 14, a broadband digital radio 16,a linear multi-carrier high power amplifier (HPA) 18 and antenna 20. Inthe illustrated embodiment the purpose of the transmitter sub-assembly10 is to accept multiple digital signals, such as those digitized voicesignals typically provided to a base station in a cellular communicationsystem from the public switched telephone network (PSTN) via a multipledigital signal transport signaling mechanism such as T-carrier (T1 orE1) or ISDN, to modulate and convert the multiple digital signals into acomposite transmitted waveform suitable for radio transmission over theantenna 20 to remote locations such as the mobile unit 30-1 or portableunit 30-2 (collectively referred to as the mobile stations 30).

The HPA 18 is typically a linear broadband multistage amplifierconnected to the output of the transceiver which amplifies thetransmitted signal to a power level required to complete a wirelesscommunication link between the transmitter 10 and the multiple mobilestations 30. For example, a transmitter 10 which is to accept ninety-six(96) input channels and make use of the standard Advanced Mobile PhoneService (AMPS) or Global System for Mobile Communication (GSM) typesignaling to communicate with mobile stations 30 located a mile orfurther distant, the multicarrier HPA must typically be capable ofproviding a total of 100 watts output power at a radio frequency in therange of 800-900 MHz over a bandwidth of approximately 5 MHz, or atleast 30 watts of output power at a radio frequency in the range of1800-1900 MHz.

The invention is a technique for reducing the intermodulation distortion(IMD) and improving the spurious free dynamic range (SFDR) of the HPA18. In order to accomplish this, the HPA 18 output is connected to theantenna 20 during normal operation but is periodically coupled to ahigh-power dummy load 21 by operating a switch 22 during a calibrationprocedure which is described in greater detail below.

A directional coupler 23 or other suitable circuit provides a sample ofthe output of the HPA 18 at a relatively low level such as -40 decibels(dB) to be used by the calibration procedure to monitor distortion inthe output waveform of the HPA 18.

The broadband digital radio 16 accepts a composite baseband digitalsignal 15 generated from the baseband digital synthesizer 12 ascorrected by the predistorter 14 and generates the transmittedmulti-carrier radio frequency (RF) signal 17 input to the HPA 18. Thebroadband digital radio consists of a digital to analog (D-A) converter160 and associated sample clock circuit 161 to drive the D-A converter160 at a relatively high speed such as 12.8 MHz, an up-converter 162consisting of a series of intermediate frequency (IF) and radiofrequency (RF) filters, and mixers (not shown) which are driven by alocal IF oscillator 163 and RF oscillator 164. The up-converter 162converts the signal energy baseband in the input composite signal 15 tothe desired RF carrier frequency in a well-known fashion.

In accordance with this invention, an additional down-convertersignaling path is incorporated into the broadband digital radio toeffectively monitor the transmitted signal 17. This is done by makinguse of the same local oscillators 163 and 164 and sample clock 161 todrive a down-converter 165 and analog-to-digital (A-D) converter 166.The result is to provide a digitized HPA output signal 150 to enableaccurate analysis of the distortion present in the up-converter 162 and(HPA) 18 while minimizing the effect of distortion in the measurementprocess itself.

Turning attention now to the input side of the transmitter sub-assembly10, the baseband digital synthesizer 12 accepts multiple digital signalsfrom an input signal source such as one or more transport signalinglines from a public switched telephone network (PSTN). A number ofmodulating digital signal processors 120-1 120-2, . . . , 120-n modulateeach of the input digital signals 121-1, 121-2, . . . , 121-m inaccordance with the modulation specified by the communication standardin use. For example, in the case of AMPS, the modulating digital signalprocessors 120 implement frequency modulation to provide a 30 kHzbandwidth baseband signal for each input digital signal 121; in the caseof GSM, the modulating digital signal processors produce atime-multiplexed 200 kHz bandwidth signal for every eight input digitalsignals 121.

The digital combiner 122 is a type of efficient filter bank synthesizerwhich makes use of multirate digital signal processing techniques togenerate a composite baseband digital signal 125 representing the nsignals output by the modulating DSPs 120 equally spaced apart incarrier frequency. The digital combiner makes use of multipleconvolutional digital filters and a Fast Fourier Transform (FFT). Apublished Patent Cooperation Treaty Publication number U.S. 95/02997(AirNet File P009) entitled "Transceiver Apparatus Employing WidebandFFT Channelizer with Output Sample Timing Adjustment and Inverse FFTCombiner for a Multichannel Communication Network" claiming a prioritydate of Apr. 8, 1994 and which was filed by AirNet CommunicationsCorporation, the assignee of this application describes the details ofseveral preferred embodiments of the digital combiner 122 in greaterdetail.

The predistorter 14 consists of a look-up table 140, a predistortionprocessor 142, a combiner sample buffer 144, and an HPA sample buffer146. The predistorter operates by taking as inputs the compositebaseband digital signal 125 generated by the digital combiner 122, thedigitized HPA output signal 150, and produces data for the look-up table140 and a calibration input signal 148.

The look-up table 140 is implemented as a random access memory (RAM)located between the output of the baseband digital synthesizer 12 andthe input of the D-A 160 in the broadband digital radio 16. The look-uptable provides a transfer function which is inverse in amplituderesponse as compared to the distortion caused by the broadband digitalradio 16 and HPA 18.

In the case of 16-bit digital composite signal 15, the RAM has 2¹⁶locations and is 16 bits wide. Additionally, in order to ensure that theHPA 18 may be operated while the predistortion processor 142 isperforming it operations, the look-up table 140 is preferablyimplemented as a pair of look-up tables 140a, 140b arranged in aping-pong fashion so that one look-up table may be written into whilethe other look-up table is in use by the transmitter sub-assembly 10.

The predistortion processor 142, which is typically a high speed digitalsignal processor such as the TMS 320 series of processors manufacturedby Texas Instruments of Dallas, Tex., performs several operations todetermine the values for the calibration look-up table and to supportnormal operation of the HPA 18. These operations include a crudecalibration procedure, a precise calibration procedure, an amplitudedistortion calibration procedure, and a normal operation procedure.

A flow chart of the operations performed in the crude calibrationprocedure is shown in FIG. 2A. Briefly, the overall idea is to storedata representing the output of the digital combiner 122 in the highspeed first-in first-out (FIFO) combiner sample buffer 144 to datameasured at the output of the HPA 18 and stored in the FIFO HPA samplebuffer 150.

In a first step 200, the switch 22 is operated to enable connection ofthe output of the HPA 18 to the dummy load 21. In the next step 201, alow level tone is driven into the HPA 18 such as by driving thecalibration input 148 to the digital combiner 122 with digital samplesrepresenting the desired calibration tone frequency and with the look-uptable values set to an initial value of zero. The calibration tone ispreferably selected to be approximately two-thirds (2/3) of the way upalong the bandwidth of the digital radio, in the example being describedfor a 5 MHz radio, this would be approximately 3.33 MHz. The tone isalso set to a level which is relatively low such that negligibledistortion effects of less than -80 dBc are present at the output of theHPA 18.

In the next series of steps, the tone is gated on and off, in step 202,followed by a step 203 where the combiner response to a particular inputvalue as represented by the output signal 125 as fed through thecombiner sample buffer 144 is compared against the HPA output 150 asprovided through the HPA sample buffer 146. By gating the tone on andoff, a crude amplitude offset can be determined by averaging thedifference between the two for each input value. In step 204, the crudeamplitude offset values are so determined.

The result of this crude amplitude offset is a first order correctionfor gain variation in the up-converter 162 and HPA 18. These effects aretypically induced by temperature and carrier frequency.

The next sequence of steps, shown in FIG. 2B are performed to determineprecise time and amplitude offsets. In a first step 205, the crudeamplitude offset values determined in step 204 are loaded into thelook-up table 140. A binary search and resampling of the HPA response isthen made to establish a time and amplitude reference. Specifically, thecalibration tone is enabled once again, in step 205, and then gated onand off again in step 207. The contents of the combiner sample buffer144 area again compared against the HPA sample buffer 146 to produce aprecise set of values for the look-up table 140, in step 208.

The combiner sample buffer 144 and HPA sample buffer 146 are typicallycloaked in synchronism with the clock circuit 161. If desired, however,more precise timing correction can be made by controlling the relativeclock phases of combiner sample buffer 144 and sample buffer 146. Forexample, if the sample rate of the D-A and A-D converters 160 and 166are set at 12.8 MHz, then crude timing procedure heretofore explainedprovides resolution to at least the nearest clock edge of 78.125 nsec.However, by allowing the predistortion processor 142 to control theexact timing offset between the two buffers 144 and 146 to 1/2, 1/4, 1/8or smaller portions of a clock cycle steps 210 and 211 can be repeatedto take additional reading to determine the position of minimum error.At this point, all calibration data is once again resampled andrecalculated (e.g., steps 207 and 208 are repeated), with this minimumtiming error offset.

A third procedure is illustrated in FIG. 2C which performs thedistortion calibration function after the precise calibration steps. Inthis procedure the precise offsets are loaded into the look-up table 140in step 212 and the calibration tone is again driven in step 213.However, this time the power level of the calibration tone is increasedin small steps (at least as small as 0.1 dB) with the contents of theHPA sample buffer 146 again being compared to the combiner sample buffer144 to derive predistortion response data. This results in a set offixed amplitude and time offsets being precisely calibrated out for thelow level undistorted input signal. Steps 214, 215, and 216 are repeateda number of times to increment the power level and measure the response,resulting in an averaged set of values that represent an estimate ofamplitude distortion as a function of combiner output level. If it isnot practical to calculate all 2 (65,536) levels, then interpolation canbe used to fill in the values for the predistortion look-up table 140which then finally represents the amplitude difference for all 65,526possible input amplitude levels associated with the digital compositesignal 15.

Finally, FIG. 2D illustrates the steps performed by the calibrationprocessor in a normal operation mode in which the HPA is connected todrive the antenna 20, where in step 220, the calibration tone isdisabled, and in step 221, the switch 22 is operated to connect the(HPA) 18 to the antenna 20.

As operating conditions change, such as temperature and number oftransmit signals and so on, data for the look-up table 140 can beperiodically and dynamically recalculated.

It is now understood how a predistortion correction of a compositedigital signal is implemented according to the invention to improve theoverall intermodulation distortion and spurious free dynamic range in anassociated multicarrier high power amplifier.

What is claimed is:
 1. In a multichannel carrier radio system such as acellular base station, a broadband multichannel transmitter assemblycomprising:digital combiner means, for generating a composite digitalsignal from a plurality of digital channel signals, the compositedigital signal representing a plurality of modulated versions of thedigital channel signals offset in carrier frequency from one another;broadband digital transmitter means, including a digital to analogconverter and up-conversion stage which accept a clock signal and localintermediate frequency (IF) and radio frequency (RF) oscillator signals,for providing a broadband transmit signal, and also including analog todigital conversion means for receiving a high power amplifier feedbacksignal and converting the high power amplifier feedback signal to adigital HPA signal; a high power amplifier, for received the broadbandtransmit signal from the broadband digital transmitter means, and forproviding a high power amplifier (HPA) output signal; predistortioncorrection and calibration means, connected to receive the digitalcomposite signal from the digital combiner means and the digital HPAsignal from the broadband digital transmitter means, to produce acorrection signal and a calibration signal; means, disposed between abroadband digital synthesizer and the broadband digital transmittermeans, for storing the correcting signal and applying the correctionsignal to the digital composite signal; and wherein the correctionsignal is connected to the input of the digital combiner means as one ofthe digital channel signals during a calibration procedure.
 2. Anapparatus as in claim 1 wherein the broadband digital transmitter meansadditionally includes a radio frequency (RF) and intermediate frequency(IF) down-conversion stage that uses the same IF and RF localoscillators that are used to generate the broadband transmit signal. 3.An apparatus as in claim 1 wherein the predistortion correction meansgenerates the correcting signal by determining a difference valuebetween the digital HPA signal and the digital composite signal.
 4. Anapparatus as in claim 3 wherein the means for storing and applying thecorrection signal is a look-up random access memory.
 5. An apparatus asin claim 4 wherein the predistortion correction means additionallygenerates the correcting signal by first loading the difference valueinto the look-up random access memory and then determining a precisecorrection signal by determining a difference value between the digitalHPA signal and the digital composite signal.
 6. An apparatus as in claim5 wherein a relative timing between the digital HPA signal and thedigital composite signal is varied to determine the correction signal.7. A process for calibrating a multicarrier high power amplifiercomprising the steps of:(a) combining a plurality of digital channelsignals and a digital calibration signal to generate a composite digitalsignal, the composite digital signal representing a plurality ofmodulated versions of the digital channel signals offset in carrierfrequency from one another; (b) generating a radio frequencymulticarrier signal by digital to analog converting the compositedigital signal to produce an analog composite signal and up convertingthe analog composite signal to a radio frequency signal; (c) couplingthe radio frequency signal to the high power amplifier; (d) correctingthe composite digital signal by adjusting the sample values thereof inaccordance with stored correction signal values; (e) determining thecorrection signal values by comparing the composite digital signal and adigital high power amplifier response signal representing a digitizedversion of the response of the high power amplifier; (f) setting thecorrection signal values to zero initially; (g) generating thecalibration signal as a tone signal of predetermined frequency; and (h)determining a set of crude amplitude offset values by gating thecalibration tone on and off and comparing samples of the resultingcomposite signals to samples of the high power amplifier output.
 8. Aprocess as in claim 7 additionally comprising the steps of:(j) storingthe crude amplitude offset values as the correction signal values; and(k) generating the calibration signal as a calibration tone to determinea precise set of amplitude offset values by comparing samples of thecomposite digital signal to samples of the high power amplifier output.9. A process as in claim 8 additionally comprising the steps of:(l)storing the precise amplitude offset values as the correction signalvalues; and (m) applying a plurality of calibration tone signals ofincrementally different amplitude to determine the amplitude offsetvalues.
 10. A process as in claim 7 additionally comprising the stepsof:(i) adjusting a time of taking samples of the high power amplifieroutput with respect to a time of taking samples of the composite digitalsignal.